The invention concerns a device for controlling the function of central lubrication installations, having at least two electrically conducting components located in a casing and integrated into a signal circuit, which components, when the lubricant is not flowing, are electrically connected to one another by means of an axially displacable contact piston located in a bore of one of the components, which piston, in the rest position, is in contact, by means of an end face, with an edge which surrounds the end of a flow duct located in one of the components and connected to an inlet for the lubricant, and which components are electrically separated from one another when the flowing lubricant lifts the end face of the contact piston from the edge of the flow duct.
A device of the above type with a cylindrical contact piston is known from the German Offenlegungsschrift No. 2,321,729, the contact piston being located with clearance in a bore of one of the components, which bore simultaneously forms a flow duct for the lubricant. In this device, a narrow annular region on the end face of the contact piston serves singly and alone to seal the flow duct in contact with the inlet for the lubricant. The known device is not fully satisfactory for several reasons. Thus, it provides no guarantee that the contact piston satisfactorily seals the end of the flow duct to be closed by it. At very small supply quantities, in particular, it must be expected that lubricant will creep past the contact piston without displacing it by an amount sufficiently large to provide a satisfactory signal. The condition for reliable and sure provision of a signal is stable interruption of the contact. This does not occur if the contact piston executes, so to speak, trembling movements forwards and backwards in the case where lubricant pulses appear. A further disadvantage is that one of the components belonging to the signal circuit is formed by a screwed pipework connection. It follows that the voltages used in the signal circuit can only be small and, as such, can contribute to erroneous indications.